The present invention relates to portable radio transceivers, such as mobile telephones. More particularly, the invention is directed at a portable radio transceiver having a transmitter power amplifier, a series transistor, which is connected to the transmitter power amplifier and is adapted, in response to a transmission control signal, to control transmission by the transmitter power amplifier, and negative bias generating means for providing negative bias to the transmitter power amplifier, where the negative bias generating means is also used for increasing a voltage applied to the gate of the series transistor so as to reduce its resistance.
The voltage supply used in portable electronic communication devices, such as mobile telephones, gets lower and lower. Output power amplification is provided by a transmitter power, amplifier, which usually comprises GaAs semiconductor components: A negative bias has to be generated and must be applied to the gates of the GaAs components, before positive supply voltage is applied. Otherwise, very high current peaks may arise and, under unfortunate circumstances, damage the semiconductor components.
A series transistor, usually a MOSFET transistor, is connected in series with the supply voltage and the transmitter power amplifier so as to disable and enable transmission by the transmitter power amplifier in response to a transmission control signal on its gate.
The series transistor must have as low internal resistance as possible during transmission, so that an unnecessary voltage drop is not generated between the battery and the transmitter power amplifier. It is well-known that the resistance of a MOSFET transistor decreases with increasing voltage between source and gate, once the conduction threshold has been reached. The battery voltage limits the highest possible voltage, that may be applied between source and gate of the series transistor, unless special means are provided.
EP-A2-0 700 165 discloses a portable radio transceiver having means in the form of a charge pump for generating a negative bias voltage to a GaAs transmitter power amplifier transistor. A positive voltage supply to the transmitter power amplifier is controlled by a series transistor (p-channel MOSFET), which is connected in series between the positive voltage supply from the battery and the transmitter power amplifier. The transmitter power amplifier is disabled and enabled is response to a transmit control signal, which is applied to the gate of the series transistor. Moreover, the charge pump for generating negative bias to the transmitter power amplifier is also used for increasing the voltage applied to the gate of the series transistor, by means of a level shifter, so as to reduce the internal resistance of the series transistor.
The portable radio transceiver shown in EP-A2-0 700 165 is thus capable of reducing the internal resistance of the series transistor, thereby reducing the voltage drop between battery and transmitter power amplifier. However, the solution shown in this patent publication requires complex circuitry for the charge pump and the level shifter.
It is an object of the present invention to simplify the circuitry required for providing the negative bias supply and the reduction in internal resistance of the series transistor for a portable radio transceiver according to the above.
Another purpose of the present invention is to provide immunity against transmit control signal faults, particularly so that a transmit control signal, which is erroneously maintained at its on-level, will not maintain the transmitter power amplifier in transmit mode during a long period of time.
It is a further object of the present invention to provide improved protection for the semiconductor components of the transmitter power amplifier.
The above objects are achieved by a portable radio transceiver according to the appended independent patent claim.
Other objects, features and advantages of the present invention will appear from the following detailed disclosure, from the drawing as well as from the dependent claims.